1. Field of the Invention
The present invention relates to a refrigeration circuit having multiple compressors arranged in series and a supplemental heat source. More particularly, the present invention concerns adding thermal energy via a supplemental heat source to the refrigeration circuit between the low and high stage compressors.
2. Description of the Prior Art
In a typical vapor compression refrigeration circuit various components such as a compressor, condenser, evaporator and expansion device are arranged to effect the transfer of heat energy between a fluid in heat transfer relation with the evaporator and a fluid in heat transfer relation with the condenser. In a heat pump system, an outdoor heat exchanger and an indoor heat exchanger are located such that the compressor, through a reversing valve, may direct hot gaseous refrigerant to either heat exchanger acting as a condenser. The other heat exchanger then acts as an evaporator such that, depending upon the position of the reversing valve, heat energy is either rejected or absorbed in both the indoor heat exchanger or the outdoor heat exchanger. In the heating mode of operation thermal energy is rejected at the indoor heat exchanger serving as a condenser and thermal energy is absorbed at the outdoor heat exchanger acting as an evaporator. The reverse is true in the cooling mode of operation wherein thermal energy is rejected at the outdoor heat exchanger acting as a condenser and thermal energy is absorbed at the indoor heat exchanger serving as an evaporator.
It has been found in air source heat pump applications that the capacity of the heat pump to provide heat energy diminishes as the ambient air temperature drops. Consequently, as the heating load is increasing, the capability of the heat pump to supply heat energy is decreasing. Many attempts have been made to increase the heating capacity of a heat pump system at lower temperatures. One of these methods is by providing two compressors in series such that the heating capacity of the refrigeration circuit may be substantially increased at lower ambient temperatures.
Another approach to increasing the ability of the heat pump to supply sufficient heat energy as the outdoor ambient air temperature decreases is to use an alternative source for supplying thermal energy to the refrigeration circuit. This approach has included bypassing the outdoor heat exchanger serving as an evaporator and routing the refrigerant to a fossil fuel fired furnace or boiler for supplying heat energy to the refrigeration circuit such that sufficient heat energy is dissipated or rejected at the indoor heat exchanger to satisfy the load on the enclosure. One of the potential disadvantages of utilizing an alternative heat source in this arrangement is that fossil fuel or electricity for supplying electric resistance heaters must be consumed to supply thermal energy to the refrigeration circuit in addition to the energy that must be supplied to drive the compressors. With the addition of heat energy to the refrigeration circuit the outdoor heat exchanger has been bypassed in the prior art devices such that the transfer of heat energy from the outdoor ambient air to the space to be conditioned is prevented.
The herein described apparatus and method utilizes a staged heat pump system having low and high stage compressors in series to avoid this problem. When a high heating load is present the outdoor heat exchanger is not bypassed. The low stage compressor continues to act to transfer heat energy from the outdoor ambient air to the indoor heat exchanger by drawing refrigerant through the outdoor heat exchanger wherein it is evaporated absorbing heat energy from the outdoor ambient air. Supplemental heating to increase the temperature of the refrigerant is arranged such that thermal energy is added to the refrigerant to increase the temperature of the refrigerant as it flows between the compressors. In other words, the supplemental heat source is arranged to increase the enthalpy of the refrigerant flowing through a quench conduit before it enters the high stage compressor. Hence, this refrigeration circuit allows heat energy to be transferred from the outdoor ambient air and from the supplemental heat source to the space to be conditioned.